Waht's the intention to apply different grade material on vessel

[duyukof]

According to the ABS rule, different part of the vessel should be applied different grade material, for example, if the whole strength deck is 30mm thick, within 0.4L amidship the deck should be 30AH, and outside 0.4L amidship should be 30DH, but 30AH and 30DH have the same thickness, yeilding point and ultimate point, and they both may service in the same enviroment (both are strength deck). So, why the rule require diferent grade material in the different parts like foregoing says?

[Ad Hoc]

Check their CVN (charpy V-notch) values, these are different.